The present invention relates to acousto-optic tunable filters and particularly those which are usable in the infrared portion of the electromagnetic spectrum. The term acousto-optic filter refers to the fact that in certain birefringent optical materials, a light beam propagating as an e-ray, can under certain conditions, be converted into an o-ray by interaction with, and diffraction from, an acoustic wave propagating in the same medium. This phenomenon has been utilized in producing narrow band optical filters the peak transmission wavelength of which can be selected by properly choosing the frequency of the acoustic wave. Such filters have typically operated in the visible spectral region using collinearly propagating acoustic and light beams in selected oxide materials. It is also known that the acoustic wave can be launched in the acoustic medium non-collinearly with the light beam to achieve the same acousto-optic narrow band filtering.
The development of new efficient infrared acousto-optic materials such as thallium-arsenic-selenide (Tl.sub.3 AsSe.sub.3) as described in U.S. Pat. No. 3,792,287, thallium-phosphorus-selenide per No. 3,929,970, and thallum-arsenic-sulfide per No. 3,799,659 all owned by the assignee of the present invention, provides the possibility of operation over the near-to-mid infrared range of from about 1.3 micrometers to about 16 micrometers. It has been the practice to utilize input and output polarizers with such acousto-optic materials to effectuate the filter capability. The conventional polarizers are either a prism type or pile-of-plates infrared polarizers which are typically available for infrared transmission. These prior art polarizers tend to limit the radiation throughput through the filter because such polarizers are only efficient for nearly normal incident light. These polarizers are also very expensive to fabricate and difficult to align with the rest of the light input and detection system with which the filter finds application.
It is also known that an efficient infrared polarizer can be fabricated by providing an array of parallel-spaced conductors upon a substrate with the spacing between adjacent conductors being less than the wavelength of infrared operation. Such spaced conductor or wire grid polarizers have found application with spectrophotometers in the spectroscopy of crystals and plastics, as elements in beam splitters and analyzers in polarizing interferometers, and in a variety of other infrared optical systems.